Confocal ultrasonic imaging system

ABSTRACT

An ultrasonic imaging system incorporates an imaging device utilizing confocal acoustic focusing of ultrasonic transducers together with apparatus for moving points of focus of the transducers to scan a volume to be imaged and gather data as to the acoustic reflectivity of matter within the volume, the gathered data being used to construct an image of the volume. In a preferred arrangement the point of focus of each transducer is moved in three dimensions. Use of confocal techniques can permit enhanced resolution to be obtained compared with conventional ultrasonic imaging techniques. In a preferred arrangement, the confocal focusing system associated with each transducer includes a pinhole which is bridged by an acoustically transparent but optically reflective diaphragm, and acoustic reflectivity data is recovered by interrogating each diaphragm by means of an optical interferometer. Optical signals from the interferometer may be scanned over an array of optical sensors such as a charge coupled device array to segment and temporarily store the optical data.

REFERENCE TO RELATED APPLICATION

This Application is a Continuation of Internation Patent ApplicationPCT/CA96/00432 Filed Jun. 26, 1996.

FIELD OF THE INVENTION

This invention relates to ultrasonic imaging systems, particularlysystems for three-dimensional imaging of soft tissue such as breast,muscle and joint tissue.

BACKGROUND OF THE INVENTION

Existing ultrasonic imaging systems have limitations upon resolutionthat restrict their usefulness in identifying fine detail. Theselimitations arise both from limited axial resolution on the Z(depth)axis due to focusing limitations, and lateral resolution on the X and Yaxes due to limitations upon the number of elements it is feasible toinclude in an array of imaging transducers, or the rate at which suchtransducers can be scanned. Axial resolution limitations can to someextent be bypassed by temporal filtering based on transit time, butlateral resolution restrictions have proved more intractable.

It is an object of the present invention to provide an ultrasonicimaging system which addresses these problems.

SUMMARY OF THE INVENTION

According to the invention in its broadest aspect an ultrasonic imagingdevice comprises at least one acoustic transducer associated with aconfocal acoustic focusing system and means to move the point of focusof each such focusing system progressively to different coordinateswithin an acoustically transmissive volume to be imaged to gather dataas to the acoustic reflectivity of matter within said volume at saiddifferent coordinates, and means to construct an image or images of thevolume from said data. Preferably multiple transducers and associatedcontocal focusing systems are disposed in an array.

By a confocal acoustic scanning focusing system is meant a deviceanalogous to the optical system of an optical confocal microscope bututilizing acoustic rather than optical energy.

Preferably multiple transducers and their focusing systems are arrangedin a two dimensional array, and are movable in two dimensions in theplane of that array so as to scan their points of focus in a planewithin said volume parallel to the array, and their points of focus arealso movable orthogonally to the array, to scan parallel planes withinsaid volume. In a first embodiment, the transducers act sequentiallyboth to transmit and receive acoustic energy through their associatedfocusing systems, but in a presently preferred embodiment, alternativemeans are used to detect reflected acoustic energy, preferably a laserinterferometer monitoring vibration of a diaphragm located at a pin-holeof the confocal focusing system.

Further features of the invention will become apparent from the appendedclaims and following description with reference to the accompanyingdrawings.

IN THE DRAWINGS

FIG. 1 shows diagrammatically a single confocal acoustic scanning devicein accordance with a first embodiment of the invention;

FIG. 2 shows diagrammatically an imaging device including an array ofsuch scanning devices;

FIGS. 3 and 4 show diagrammatically how the imaging device according toclaim 2 may be utilized tomographic imaging;

FIG. 5 is a diagram illustrating a single confocal acoustic scanningdevice in accordance with a second embodiment of the invention;

FIG. 6 illustrates a first technique for providing an array of devicesas shown in FIG. 5;

FIG. 7 illustrates a second technique for providing an array of devicesas shown in FIG. 5; and

FIG. 8 illustrates a modification of the embodiment shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, an acoustic transducer 2 acts alternately asboth an acoustic transmitter, transducing into acoustic energy pulses ofhigh frequency electrical energy from a driver 4 under control of acontrol unit 6, and an acoustic receiver, receiving reflected acousticenergy at the same frequency and transducing it to electrical energy,which is preamplified in a preamplifier 8 and passed via an analogsignal preprocessor 10 to a signal processing computer 12. Thetransducer is associated with a first acoustic lens 14, which may beintegral with the transducer if the latter has an appropriately formedradiating surface, which lens focuses energy from the transducer on asmall aperture or pin-hole 16 in an aperture plate 18. Acoustic energyemerging from the pin-hole is focused by a second lens 20 to form animage of the pinhole at a focal point 26 in a focal plane within anacoustically transmissive body 22, typically soft tissue as mentionedabove. The body 22 may be matched acoustically to a medium such as wateror other liquid or gaseous medium within which the remainder of theacoustic system is located, by a coupling layer or membrane 24. Assumingthat the lens 20 has a large aperture, that the pin-hole is very small,that the performance of the lens is good, and that the frequency of theacoustic energy is high enough that its wavelength is very smallrelative to the optical dimensions of the components of the system,acoustic energy from the transducer will have a sharp intensity peak atthe focal point with good lateral and depth resolution, and equally,energy reflected from that point will be focused by the lens 20 on thepin-hole 16, with a high degree of rejection of acoustic energyreflected from elsewhere within the body 22. Reflected acoustic energypassing through the pin-hole, and in this embodiment passing through thelens 14 to be received by the transducer, is therefore proportional tothe reflectivity of the body 22 at the location of the focal point, andthe intensity of echo signals from the preamplifier 8 resulting from aseries of pulses applied by the driver 4 will vary according with theacoustic reflectivity of the body 22 as the focal point 26 is movedrelative to the body, for example by moving the scanning device as awhole.

The device shown in FIG. 2 consists of a planar array 30 of confocalsystems as described with reference to FIG. 1, mounted within a housing31 for motion in three-dimensions within the housing under control ofactuators 33, 35 and 37 powered by electrical driver circuits 39. Theacoustic transmission medium within the housing is acoustically coupledto the body 22, for example human tissue to be imaged, through thecoupling layer 24 in the form of a flexible membrane. The scanning arraycomprises a set of systems comprising acoustic transducers 2 andconfocal focusing systems having lenses 14 and 20, and pin-holes 16, thecomponents being arranged in planar lattices. The focusing systems inthe array 30 have focal points 26 in an image plane 32 which is aprecise distance from the array 30. The scanning device elements in theplanar array are identical and define an image plane within the body 22.

The use of ultrasonic acoustic energy in such a contocal system canallow a substantial improvement of both the lateral and axial resolutionof an ultrasonic imaging instrument. By overcoming the resolution limitsof conventional ultrasonic systems the confocal arrangement is capableof lateral precision comparable to standard X-ray techniques andimproves upon their axial resolution, but does not make use of ionizingradiation. Moreover, the complete instrument does not require the largemagnetic fields associated with magnetic resonance imaging (MRI) nor theshielding safeguards of X-ray examinations, and so offers advantages inboth cost and convenience for clinical tomographic imaging. Furthermore,the arrangement is well adapted to exploit modern piezoelectric plasticsalong with surface mount and photolithographic fabrication techniques toprovide a compact and economically manufactured instrument.

An image of body tissue 22 or other acoustically transmissive materialis generated in stages using the apparatus of FIG. 2. First, in atranslational stage, the scanning array 30 is moved in small steps (afraction of the spacing of the axes of the focusing systems in thearrays) in its own plane between each pulse. At each step the acoustictransducers 2 individually generate a brief coherent pulse of ultrasonicenergy at a predetermined frequency which is focused by the lens systemto a point within the body 22. The translational motion, over an areashown in FIG. 1 at 32, is preferably in a “dither” pattern 25 designedso that the systems in the array will together completely map the tissuein the image plane at a desired lateral resolution, with each systemmapping an area 27 determined by the pitch of the array. In the nextstage, the scanning array is advanced incrementally in a directionperpendicular to the acoustic axes of the focusing systems, and afurther translational stage follows in order to map an adjacent parallelimage plane of the tissue. In this manner successive layers of thetissue, each with an area equal to that of the scanning array, areimaged at lateral and axial resolutions corresponding to the ditherincrement and the perpendicular advance increment. The signals receivedby the individual acoustic transducers of the scanning array arediscriminated from each other by the confocal nature of the ultrasonictransducer lens system which has the effect of increasing lateral andaxial resolution while limiting out-of-focus noise. In addition, bytime-of-flight techniques involving only accepting received signalsduring a temporal window commensurate with the expected arrival time ofa return signal, extraneous signals and cross-talk between the acousticelements of the array can be further reduced. By monitoring frequencyshifts of the received signals, provision can be made for the use ofdoppler techniques to study fluid flow within tissue, and differentultrasonic frequencies may be utilized to obtain optimal discriminationof tissue types. Through computer processing the data developed duringdithering of individual devices within the array can be rearranged anddisplayed utilizing a conventional raster-type scan, allowingtomographic images to be regenerated using existing imaging software.These and other known computational procedures may be utilized toenhance and assemble the images into a three-dimensional volume forclinical viewing.

The total time for a complete tissue scan is significantly reduced bythe scanning array system. If, for example the array 30 consists of 8×10devices with centres 1 cm apart, there are 25 steps in the ditherpattern, and there are 50 translational increments perpendicular to theacoustic axes of the scanning array of 0.2 cm, then an image of a tissuevolume of 8 cm ×10 cm ×10 cm at a volume resolution of 0.008 cm³ isproduced. If the time for an acoustic pulse to be sent and received andfor the translational actuators to move the scanning array through onedither increment is approximately 1 ms, the entire volume can be imagedin about 1.25 seconds. This rapid processing rate makes it feasible toutilize images generated by the system to guide the insertion ofsurgical instruments such as biopsy needles.

FIGS. 3 and 4 illustrate two ways in which the scanning array describedabove can be applied. In FIG. 3, the array 30 is mounted on aspring-loaded extensible strut 40 in turn movable around a quadrant 42itself mounted at one end to a circular track 44. By including suitableactuators and displacement transducers in the various mountings, thehousing 31 of the array 30 can be accurately positioned within ahemispherical volume under control of the control unit 6. Thus apressure-sensitive sensor incorporated in a continuous or discretefeedback system controls the extension of the strut 40 to maintain afixed position on the surface of the tissue to be imaged. The entiredevice is then stepped over the surface of the tissue to be imaged in ahemispherical envelope with the step size equal to the width of thescanning array 30, which performs a scan at each step. The transducersprovide data to locate the position of the array and its orientationprecisely so that a three-dimensional image of the tissue can be builtup without distortion by combining images formed at each step.

In FIG. 4, the scanning array 30 is shown mounted to a hand-held support50. An operator uses the support to bring the coupling membrane 24 tothe surface of the tissue being imaged and holds it there while thescanning array completes its tomographic scan. Springs 52 and 54 act topress the array, universally mounted by a ball and socket joint 56 on arod 58 journalled within the support 50, and a sensor 60 coacting withthe rod determines when appropriate pressure has been applied to thearray by the operator so as to enable scanning.

Although the resolution of conventional ultrasonic imaging systems isrelated to the frequency utilized, and pulse width, the essentiallyechographic nature of such systems typically limits their lateralresolution to about 5 millimetres, whereas using the confocal principlesof the present invention, lateral resolutions condensed by about anorder of magnitude should be obtainable, thus enabling recognition ofmuch smaller features, and rendering ultrasonic techniques suitable formammography. Transducers may be fabricated from machinablepiezo-ceramics, but preferably piezoelectric plastics are utilized sincethese are particularly well adapted to the fabrication of arrays.Similarly, the acoustic lens and pin-hole arrays utilized are readilyfabricated as plastic mouldings which are of relatively low mass andthus permit the array to be stepped rapidly within its casing byvoice-coil or other suitable actuators capable of rapid stepping rates.An exemplary transducer element within the array may have a diameter ofabout 10 mm, and operate at a frequency of 20 MHz, and a power densityof more than about 50 mW/cm², so as to avoid any risk of excessive powerdissipation in tissue being examined.

Although the embodiment described uses the same transducers to bothtransmit and receive acoustic energy, a separate transmitter ortransmitters could be utilized to insonify the tissue to be scanned, butthere would be significant loss of the advantages of the confocalarrangement since the transmitted energy would not be concentrated atthe focal points of the confocal focusing systems, thus reducing theintensity of the reflected signals and increasing background noise. Thedithering motion of the array during scanning could be continuous, sincedisplacement of the points of focus during actual reflection of a pulsewill not usually be significant. It would also be possible to displaceonly parts of the focusing system during dithering, or to cause relativedisplacement of the parts, in order to provide a desired shift in thepoint of focus. For example, it may bepossible in some cases to moveonly the second stage lenses or the aperture plate provided that auniform level of insonification of the points of focus can bemaintained.

Referring to FIGS. 5-8, FIG. 5 illustrates the operating principles of asecond embodiment of the invention, in which an alternative technique isemployed to sense acoustic signals passing through the pin-hole of aconfocal acoustic focusing system generally as described above withreference to FIG. 1. Thus the system comprises an acoustic transducerelement 2, a lens 14, a pin-hole 16 formed in an aperture plate 18 andan acoustic lens 20. The pin-hole 16 is however covered by a membrane100 formed of a film which is acoustically transparent but opticallyreflective, at least on one surface. A thin metallized synthetic plasticmembrane will provide the cleared properties and vibrate in sympathywith acoustic waves traversing the pin-hole. If a light beam isreflected off a reflective surface of the diaphragm 100, then the pathof the light beam will be modified as the membrane vibrates, providing ameans to monitor the passage of acoustic energy through the pin-hole 16.While numerous techniques are known for monitoring deflection of lightbeams by moving reflective surfaces and could be employed, a presentlypreferred technique for monitoring vibration of the diaphragm 100 is tomonitor changes in the length of the reflected light path by means ofoptical interferometry. While the technique to be described is based onthe well known Michelson interferometer, other suitable types of opticalinterferometer could be utilized. Optical interferometers require asource of coherent light, conveniently provided by a laser 102. A lightbeam from the laser is split by a beam splitter 104, conveniently ahalf-silvered mirror, which splits the beam into two portions 106 and108, the first of which is impingent on a reflective surface of thediaphragm 100, and the second on a mirror 110, the reflected beams beingrecombined by a beam combiner 112, conveniently again a half-silveredmirror into a single beam impingent on an optical sensor 114.Conveniently both the laser 102 and sensor 114 are semiconductor devicessimilar to those widely used in compact disk players and the like. Whenan acoustic pulse passes through the diaphragm 100, the latterreproduces the movements of the acoustic medium to either side of thediaphragm such that it is acoustically transparent, but the resultingmovements of the diaphragm produce charges in the path length of thebeam portion 106 which do not occur in the beam portion 108. Thesechanges in path length result in interference between the recombinedbeams.

The electronic design of the system can be simplified since it is merelynecessary to detect fluctuations in the intensity of coherent lightimpingent upon the sensor 114, which in turn result in amplitude changesin the output of the sensor which reproduce the acoustic signals passingthrough the diaphragm 100. The sensitivity of the system is governed bythe angle of incidence of the interrogatory beam 106 on the diaphragm100 and the wavelength of the radiation from the laser.

The single channel system of FIG. 5 may be converted into a multiplechannel system simply by multiplying or enlarging the area of thecomponents as shown in FIG. 6, where multiplied components have the samereference numbers as in FIG. 5 but with an A or B suffix according tothe channel to which they belong. While only two channels are shown inFIG. 6 for clarity in illustration, the number of channels can beincreased, and the multiple components disposed in two dimensionalarrays. The highly directional nature of laser beams, and the lack ofcoherence between the individual lasers can render such a systempossible without excessive cross-talk between channels.

Economy of components and greater ease of alignment may be achieved asshown in FIG. 7 by utilizing only a single laser 102, in conjunectionwith holographic beam splitters 116 and 118. The beam splitter 116produces a plurality of parallel beams 106A, 106B which are incident onthe diaphragms 100A and 100B, while the reference beam 108 is split byan identical splitter 118 prior to recombination. This produces a systemwhich can be more compact and easier to align since it uses only asingle laser and a number of components which does not increase numberof channels.

Referring now to FIG. 8, the system of FIG. 5 is modified by theaddition of a rotating or oscillating mirror 120 driven so as to sweepthe recombined beam across an array 122 of light sensors such as a CCD(charge coupled device) array. Such an optically swept system permitsimproved frequency response of the system without requiring very fastcomponents in the sensor circuits. The CCD array acts as a fast memoryfor high frequency fluctuations in the optical beam amplitude, which maybe subsequently accessed and evaluated utilizing much slower electroniccircuits. Multiple channel versions of this embodiment may be providedusing techniques similar to those described in FIGS. 6 and 7.

The embodiments of FIGS. 5-8 provide several advantages over theembodiment of FIGS. 1-4. The interferometric system provides a muchgreater sensitivity to acoustic vibrations for reasons discussed above,while acoustic losses occurring between the pin-hole and the acoustictransducer are eliminated. The system has greater noise immunity sinceit is immune to ambient acoustic noise present in the transducer orgenerated in preamplifiers coupled to a reciprocal acoustic transducer,and noise fluctuations in the optical detectors cannot generate acousticnoise at the pin-hole.

The interferometric sensing system allows a much faster response sincethe relatively massive acoustic transducer is replaced as a sensor by adiaphragm at or adjacent the pin-hole which can be very light in weight.Furthermore, it is not necessary to blank or disable the receiverelectronics during ringing of the acoustic transducer after it generatesan acoustic pulse, which provides greater freedom in transducer designand allows higher amplitude acoustical pulses to be used for imaging.The optically swept embodiment of FIG. 8 in particular provides animproved frequency response.

There is no need for the transducer to be connected to both transmitterand receiver circuits, with associated switching and/or blankingcircuits, and the necessity to handle vastly different signal levelsduring transmission and reception.

It will be appreciated that in the embodiments of FIGS. 5-8, the signalsfrom the light sensors are passed to a preamplifier 8, preprocessor 10and a computer 12 as shown in FIG. 1 and the remainder of the system maybe generally as described for example with reference to FIG. 3 or 4. Thecommencement of a pulse may either be signalled by a control unit 6, ordetected by the light sensors which can of course sense the passage ofboth outward and reflected pulses.

The interferometric interrogation system described above may also beutilized to advantage in other pulse-echo acoustic systems in which atarget is insonified by pulses of acoustic energy and the acousticenergy reflected from the target is monitored to provide data. Byplacing an acoustically transparent but optically reflective diaphragmbetween an acoustic transducer used to generate the pulses and thetarget, the reflected acoustic energy may be monitored without needingto utilize the acoustic transducer for this purpose, with many of theadvantages set forth above.

What is claimed is:
 1. An ultrasonic imaging device comprising at leastone acoustic transducer each transducer being associated with its ownacoustic focusing system, a means to move the point of focus of eachfocusing system progressively to different coordinates within anacoustically transmissive volume to gather data as to the acousticreflectivity of matter within said volume at said different coordinates,and means to construct an image or images of the volume from said data,each focusing system being a confocal acoustic focusing system includinga first acoustic lens associated with the transducer, an aperture platehaving a pinhole upon which the first lens is focused, and a secondacoustic lens on an opposite side of the aperture plate to the firstlens and focused upon said pinhole and said point of focus respectively,wherein an acoustically transparent and optically reflective diaphragmis applied spanning each pinhole, and each diaphragm is associated withan optical monitoring means generating a beam incident upon andreceiving the beam as reflected by the diaphragm, to interrogatemovement of the diaphragm responsive to passage of acoustic energytherethrough, whereby to detect acoustic energy reflected to the pinholefrom the point of focus.
 2. A device according to claim 1, comprisingmultiple acoustic transducers and their associated confocal focusingsystems, disposed in an array.
 3. A device according to claim 2, whereinthe array is a two-dimensional planar array.
 4. A device according toclaim 3, wherein the means to move the points of focus includesactuators to move the array on two perpendicular axes parallel to theplane of the array and a third axis perpendicular to the plane of thearray.
 5. A device according to claim 4, wherein the actuators movingthe array in its plane do so in a dither pattern of dimensions inaccordance with the spacing of the axes of the focusing system in thearray.
 6. A device according to claim 1, wherein each transducer is anelectroacoustic transducer connected to a driver for applying pulses ofelectrical energy at an ultrasonic frequency to the transducer to causeit to transmit pulses of acoustic energy.
 7. A device according to claim1, including a membrane providing a coupling layer between said at leastone focusing system and said volume to be imaged.
 8. A device accordingto claim 1, wherein each monitoring means is an optical laserinterferometer.
 9. A device according to claim 8, including an array ofconfocal acoustic focusing systems and their associated interferometers.10. A device according to claim 9, including an array of confocalacoustic focusing systems, and wherein the associated interferometersshare a single laser having a beam splitter in an interrogation beampath providing an array of parallel beams, one to interrogate each ofthe diaphragms associated with each of the array of confocal acousticfocusing systems, a beam splitter in a reference beam path to provide anarray of parallel beams for recombination, one with each of thereflected parallel beams of the interrogation path, and an array ofoptical detectors one to receive each of the recombined beams.
 11. Adevice according to claim 9, including an array of optical detectors,one array for receiving optical signals from each of the diaphragmsinterrogated and means for scanning the optical signals over each arrayto direct successive samples of the received optical signals tosuccessive detectors of the array.
 12. A device according to claim 11,wherein each array of optical detectors is a charge coupled arraystoring the samples for later recovery.
 13. A pulse-echo acoustic systemincluding an acoustic transducer utilized to insonify a target withpulses of acoustic energy transmitted along a path from the transducerto the target, and means to monitor acoustic energy reflected back fromthe target along the same path, wherein the monitoring means comprisesan optical interferometer in which an acoustically transparent butoptically reflective diaphragm, which is located on the path between thetransducer and the target, forms a reflective element in an opticalinterrogation beam path of the optical interferometer.
 14. A pulse-echoacoustic system according to claim 13, wherein the path from thetransducer to the target incorporates an acoustic focusing system.
 15. Apulse-echo acoustic system according to claim 14, wherein the acousticfocusing system is a confocal acoustic focusing system.
 16. A pulse-echoacoustic system according to claim 13, wherein the diaphragm spans apinhole of the confocal acoustic focusing system.